The bug still occurs with driver 256.44.
It also occurs, but much less frequently, in Mandriva 2010.0 (Xorg v1.6.5).
When installing x11-server-xorg and x11-driver-input-* v1.4.2 (from Mandriva 2009.0) into Mandriva 2010.1, the bug doesn't occur any more.

In Mandriva 2010.1 with Xorg v1.7.7, I also have strange delays (up to 2s !) on GTK2 windows redraws when switching desktops or starting the GTK2 app (in particular with gFTP and ossxmix) during which the CPU is loaded at 100%, and this with all NVIDIA driver versions I tested (from 195 to 256). This delay doesn't happen in Mandriva 2010.0 (Xorg v1.6.5) neither Mandriva 2009.0 (Xorg v1.4.2).

From the above I can conclude that there is definitely something wrong between NVIDIA drivers and Xorg v1.6 and v1.7 (with the latter having the worst issues).